Transformation and Agronomic Use of Nutrients From Digester Effluent

Table of Contents

Anaerobic Digestion Nutrient Transformations

Anaerobic digestion (AD) is the process in which organic compounds are broken down by naturally occurring bacteria, including methanogenic microorganisms under oxygen free conditions, transforming organic matter into biogas (methane (CH4), carbon dioxide (CO2), water vapor, ammonia, and hydrogen sulfide) (Dugba and Zhang, 1999; AgSTAR, 2010b). The end products of this process include biogas, a renewable energy source, and treated manure containing plant nutrients that can be used to replace agricultural fertilizers. The nitrogen, (N), phosphorus (P), and potassium (K) are not lost or reduced due to the AD process, but are transformed (see figure 1) from organic forms to inorganic forms while the carbon is converted to biogas (Table 1). As a result, the levels of ammonium N and inorganic P increase as a percent of total N and total P when compared to raw manure. The increase in ammonium content will vary due to pre-digester management of the manure. Data from Washington State University indicates that ~ 40% of the N going into the AD is NH4+-N, and 60 % of N coming out of the AD is NH4+-N (O’Rourke et al, 2009 SWCS presentation)

Figure 1. Transformation of nutrients passing though digester

Chemical Composition of Digested Effluent

Nitrogen

Nitrogen that enters a digester from dairy manure is either in the ammonium or organic form. Much of the organic nitrogen is converted via nitrogen mineralization during the digestion process to ammonium, raising the overall level of ammonium in the effluent (Field et al., 1984). Although a small amount of ammonia gas will be lost to biogas, the total nitrogen leaving the digester is generally considered equal to that added to the digester. (Topper, 2006).

Nutrient content of the AD input will vary depending on the species of the contributing manure and if there is any addition of other organic feedstocks (co-digestion). Kirchmann and Witter (1992; Table 1) evaluated fresh and anaerobically digested manure from three different species for nutrient concentration. They conclude that anaerobic digestion of manure resulted in higher ammonium N concentrations (50-75% of total N) in the digested material. In a similar project with co-digestion of dairy manure and pre-consumer waste feedstocks, they observed an increase from 34% NH4 (% of total N) in pre-AD material to 58% NH4 (% of total N) post-AD material prior to liquid-solid separation  (7.3 NH4 and 21 total N lbs./ 1000gal before digestion; 9.1 NH4 and 15.8 total N lbs./1000 gal post digestion, Whitefield, 2009). Anaerobic digestion facilitates nitrogen mineralization, while carbon is converted to biogas.  Additionally, carbon is partially removed from the digested material,  reducing the C:N ratio (Kirchmann and Witter, 1992; Moller et al., 2008).

Table 1. Forms of nitrogen in fresh manure and anaerobic digestion effluent (Kirchmann et al., 1992; numbers in parenthesis are the percent of total N).

Phosphorus

Nutrient speciation data collected from previous AD studies suggest that a high percentage of the P can be found in the inorganic form in the AD effluent (Wrigley et al., 1992; Bowers et al., 2007; Marti et al., 2008; Moody, 2009). Moody (2009) and colleagues demonstrated a 26% increase of inorganic P (PO43) in digested swine slurry compared to the raw swine slurry (1591 mg/L and 1256.2  of PO43– respectively). Inorganic P is comprised of soluble and insoluble orthophosphates and polyphosphates. When evaluating the nutrient transformation of five different types of ADs in New York, the percent change in orthophosphorus (OP) after digestion varied from 7-27% depending on the type of AD (Figure 2). The case study data from Cornell (Figure 2) also demonstrates the percent change of total Kjeldahl nitrogen (TKN), ammonia nitrogen (NH3-N), organic nitrogen (ON), and TP (Gooch et al, 2006). The positive percent change indicates a greater concentration of the nutrient in the post digested effluent compared to the influent vs. a negative percent change which indicates the nutrient is more concentrated  in the influent before digested compared to after digestion.  These data represents several farms with digesters, some with different digester models, and therefore variation would be expected. Bowers et al., (2007) demonstrated total phosphorus (TP) content ranging from 238 to 323 ppm, from which OP contributed 106 to 231 ppm of the TP in the post digestion effluent of a co-digestion dairy manure AD. Also one should not expect loss of N or P during digestion, and variability due to sampling and analyses could have caused some error in mass balance calculations.

Figure 2.  Nutrient Transformation (% change) of five different farms with digesters from a case study in NY state (Source:  Gooch et al, 2006) The percent changes was calculated as influent nutrient value minus effluent nutrient value; therefore even though there is a negative percent change of NH3-N and OP, there is a greater amount in the digested effluent compared to the influent.

pH and Chemical Oxygen Demand

The pH of the manure remains fairly neutral throughout digestion maintaining microbial stability within the digester (Wen, 2009). In a study by Wang and collegues (2010) anaerobic digestion reduced manures’ chemical oxygen demand and amount of total and volatile solids by 30-40%.

Organic nitrogen is mineralized to ammonium while conserving total nitrogen and phosphorus (Wang et al., 2010). The Danish Biogas Institute reports 25% more available NH4-N and higher pH in AD manures (Monnet, 2003).

Liquids-Solids Separation

Many AD systems are managed with the use of liquids-solids separation after the manure has been digested. Separating out the solids for use as a soil amendment or bedding can result in a small reduction of nutrients in the remaining fraction. Preliminary data from Washington State University suggests that 27 % of the solids, 6 % of the N, and 8 % of the P are removed in the solids from AD treated manure (screw type solids separator).

Potential for Increased Efficiencies

Although there is a growing body of research on anaerobic digestion and crop nutrient availability, this technology has not been extensively studied. Most work has been focused on short term nutrient recovery (1-3 years), and not the long term impacts (5+ years) of fertilizing with AD manures (Arthurson, 2009).

AD manure has been shown to have the same positive effects on yield and crop production when applied at equal rates of plant-available N as synthetic fertilizers or raw manures in corn and forage production systems (Morris and Lathwell, 2004; Loria et al., 2007), while soil quality and fertility indicators are improved relative to synthetic fertilizers (de Boer, 2008; Arthurson, 2009).

Since anaerobically digested dairy manure could provide more plant available nitrogen than untreated manure (Kirchmann and Witter, 1992; Michel et al., 2010), the potential exists for increasing agricultural efficiences (Morris and Lathwell, 2004; Moller and Stinner, 2010). Increased concentrations of NH4-N in AD manure would increase the potential for N loss in the field, so best management practices would be required to take advantage of the higher NH4-N content.

Application of AD dairy manure to corn has been shown to produce similar total plant N uptake and equivalent or greater yields than inorganic fertilizer.  Early growth/yield of corn was greater from application of AD dairy manure than from synthetic fertilizers on acidic soils (<7.0) but not on alkaline soils (Morris and Lathwell, 2004). The acidic soils restrict ammonia loss from ammonium-rich AD manure resulting in greater N uptake (Nelson, 1982).

Figure 3 summarizes information from 2 years of a study (Saunders, 2011) conducted to look at anaerobically digested dairy manure or undigested dairy manure. When anaerobically digested or undigested manure was applied at equal amounts of total nitrogen, equivalent amounts of dry matter yield (~ 7.5 tons) and similar amounts of nitrogen uptake (~ 475 pounds) were observed. The control did not receive any form of fertilizer or manure, and was stastically different (P<.05) from the manure treatments before and after digestion.

Figure 3. Average annual yield of dry matter and annual nitrogen uptake by grass receiving equal amounts of nitrogen from anaerobically digested dairy manure or undigested dairy manure.

Manure serves as a useful, low-cost source of nutrients for crop production (Sommerfeldt et al., 1988; Jokela, 1992; Ferguson et al., 2005; Nyiraneza and Snapp, 2007). Anaerobically digested manure provides sufficient nutrients to support biomass and crop yields equivalent to synthetic fertilizers and raw manures (Bittman et al., 1999; Loria et al., 2007). Some studies, (Rubaek, 1996; Chantigny et al., 2007; de Boer, 2008) have found increased yield and nitrogen availability with application of anaerobically digested material as compared to non-digested material, possibly due to increased nitrogen content and reduced carbon content, which can result in nitrogen mineralization by microbes. In addition, manure applications to soils have enhanced soil quality and fertility compared to soils receiving synthetic fertilizers (de Boer, 2008; Arthurson, 2009). A crop will typically recover <50% of applied fertilizer nitrogen (Stevens et al., 2005). Up to 46% of applied manure nitrogen may be left over in the soil at the end of the growing season, increasing the potential for loss, after multiple applications during a season (Munoz et al., 2003). Over-application of manure nitrogen in excess of crop uptake can result in nitrate leaching (Angle et al., 1993). Some studies have indicated that manure nitrogen poses an equal or slightly less risk to leaching than synthetic fertilizers (Jokela, 1992;Trindade et al., 2009). Others have determined manure increases nitrate leaching (Jemison and Fox, 1994). During winter months when plants are dormant, nitrate leaching can be the main source of N loss (Bakhsh et al., 2007). The shift in organic to inorganic nutrients during the AD process should be considered when developing a farm nutrient management plan.

References

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  •  Sandy Anderson, Washington State University

Pathogen Reduction in Anaerobic Digestion of Manure

Benefits of Anaerobic Digestion of Manure in Reducing Pathogens

Manure is a biologically active material that hosts and supports many microorganisms and thus can seldom be considered “pathogen free.” Certain manure handling techniques and methods, however, can limit the production and multiplication of such pathogens. Common sense must be used when making manure handling decisions. Pathogens are microbes such as bacteria, viruses, protozoa, and other organisms that cause disease. These pathogens persist commonly in animal manures. For more information about pathogens and zoonotic pathogens, see Pathogens and Potential Risks Related to Livestock or Poultry Manure. A list of animal related microorganisms (including some that are pathogens) are listed in Table 1.

Table 1. Animal Related Microorganisms
Fecal coliforms (an indicator bacteria, not all coliforms are pathogenic)
Salmonella spp. (pathogen)
Generic E. coli (not all E. coli are pathogens), including O157:H7 (pathogen)
Enterococci (not generally considered pathogenic)
Listeria (pathogen)
Clostridium (pathogen)
Mycobacterium paratuberculosis (MAP or Johne’s) (pathogen)
Enterovirus (pathogen)
Campylobacter (pathogen)
Cryptosporidium (C. parvum is the only one related to animal manure that is considered pathogenic)
Bovine Spongiform Encephalopathy (BSE) (The prions that cause BSE are not a true pathogen, but are considered an “infectious agent”)

Excessive or careless land application of manure and livestock facility runoff can contaminate surface water. This manure laden runoff can pose significant risk to human and animal health. Stored or fresh manure can be applied to land with minimal reduction of harmful pathogens, as some microorganisms can persist for long periods outside an animal’s body.

Treatment through anaerobic digestion can greatly reduce the number of pathogens within the manure and therefore limit the number of pathogens entering the environment. Anaerobic digestion (AD) of manure has a pathogen reducing effect with as much as 95-98% of common pathogens eliminated in mesophillic (~ 100 degrees Fahrenheit) digesters. The reduction in pathogens has the potential to be of benefit for: manure application in impaired watersheds when trying to manage certain pathogens such as Mycobacterium paratuberculosis (MAP or Johne’s) or salmonella, and when considering a community- based anaerobic digester where manure from multiple farms is combined, treated, and AD solids and AD effluent returned back to the farms.

Supporting Research-What We’ve Learned

There is a growing body of research which demonstrates the anaerobic digestion process can vastly reduce if not eliminate the concentration or presence of numerous organisms. Current research in this area is summarized below in Table 2.

Table 2. Potential for microbial (including pathogen) and infectious agent reduction by anaerobic digestion
Microbes Reduced By Anaerobic Digestion Microbes Not Reduced By Anaerobic Digestion
Salmonella Bovine Spongiform Encephalopathy (BSE) (Infectious agent–not a microbe)
Generic Escherichia coli  
Escherichia coli O157:H7  
Mycobacterium paratuberculosis (Johne’s)  
Bovine enterovirus (BEV)  
Enterovirus  
Fecal coliform  
Cryptosporidium  

Anaerobic digestion of manure has been shown to reduce the Johne’s-causing organism, Mycobacterium avium a subspecies of paratuberculosis. Thermophilic digesters operating at 135 degrees F. have shown complete elimination of Johne’s bacteria, while digesters operating at 99 degrees F with a 20-day retention time have demonstrated significant reduction [3]. Other potentially harmful pathogens to humans include Escherichia coli O157:H7, Salmonella, and the protozoan parasite Cryptosporidium parvum. These bacteria and protozoa have all been reduced in number of viable and infectious organisms after passing through a digester. Pathogen reduction of 95% is possible with a 20-day retention time under mesophilic conditions (95-105 degrees F.) with a digester [3].

Anaerobic digestion under mesophilic or thermophilic conditions has not been shown to reduce or eliminate Bovine Spongiform Encephalopathy (BSE), or Mad Cow Disease. Although little is known about this disease, it is accepted that the protein-infecting prions are resistant to heat. Even thermophilic conditions (135 degrees F.) are not sufficient to destroy BSE prions [3].

In a study in New York state, samples were taken from a plug-flow digester over a 14-month period and tested for fecal coliform and Mycobacterium avium paratuberculosis (MAP), or Johne’s disease. It was found (see Table 3) that anaerobic digestion has the potential to reduce the number of fecal indicator bacteria in dairy effluent, including in this study, by 100% reduction of MAP CFU/gram. The substantial reduction of pathogen concentrations led the authors to recommend anaerobic digestion of dairy manure when concentration of pathogens is a concern [4].

Table 3. Pathogen results from dairy manure treatment
  Fecal coliform CFU/Gram MAP CFU/Gram
Raw Manure 3,836,000 20,640
Digested Effluent 3,400 136
Wright et al. 2001

In a study conducted by Washington State University on two operating anaerobic digesters in Oregon (2004), pre-digested and post-digested samples were taken bi-weekly, for six sampling events. Samples were obtained from: manure prior to the AD system, and solids and liquids post-AD. The design of the two digesters was different: one was a plug-flow and the other, a continuous mix, each operating at 100 degrees F. and with expected retention times of ~ 21 days and 24 hours, respectively. Specific organisms selected for evaluation were: Salmonella, Generic E. coli (including 0157:H7), enterococci, Mycobacterium paratuberculosis (Johne’s), and enterovirus.

 

Figure 1. Generic E.Coli concentration in anaerobic digester samples


 

Figure 2. Enterococci concentration in anaerobic digester samples


The data indicated reductions in fecal indicator bacterial concentration was > 98% (generic E. coli, enterococci, and enterovirus) in most cases (see figure 1 and 2). While the detection of Mycobacterium paratuberculosis was reduced in post digested samples, greater than 50% of samples had detectable levels. The data from this study suggests that AD treatment of dairy manure does not completly remove all biosecurity hazards [2].

Additional Resources

Bibliography

  1. Spiehs, Mindy; Goyal, Sagar. Best Management Practices for Pathogen Control in Manure Management Systems. University of Minnesota Extension. 2007.
  2. Harrison, J.H., D. Hancock, M. Gamroth, D. Davidson, J.L. Oaks, J. Evermann, and T. Nennich. 2005. Evaluation of the pathogen reduction from plug flow and continuous feed anaerobic digesters. Symposium – State of the Science Animal Manure and Waste Management. San Antonio, TX. Jan. 5-7
  3. [3.0][3.1][3.2]Topper, Patrick; Graves, Robert; Richard, Thomas. The Fate of Nutrients and Pathogens during Anaerobic Digestion of Dairy Manure. Penn State Cooperative Extension. Agriculture and Biological Engineering. Extension Bulletin. 2006.
  4. Wright, P. E., S. F. Inglis, S. M. Stehman, and J. Bonhotal. “Reduction of selected pathogens in anaerobic digestion.” 5th Annual NYSERDA Innovations in Agriculture Conference (2001): 1-11.
  5. “Pathogen Overview.” Information Collection Rule. US Environmental Protection Agency, 10 Apr. 2009. Web. 7 Dec. 2009.

Contributors to this Article

Authors

  • Olivia Saunders, Crop and Soil Science, Washington State University
  • Joe Harrison, Professor, Nutrient Management Specialist, PAS, Washington State University

Peer Reviewers

Separation Technologies for Capturing Nutrients from Manure

Exporting phosphorus and possibly nitrogen from larger livestock operations as well as regions of large livestock populations is often essential for protecting water quality. Solids (and nutrient) separation technologies are an option for concentrating nutrients for export. This webinar introduces three approaches to solids separation that are being applied in commercial settings. This presentation was originally broadcast on January 18, 2019. More… Continue reading “Separation Technologies for Capturing Nutrients from Manure”

Linking Feed Management to Whole Farm Nutrient Management

Nutrients in feed is the primary source of Nitrogen, Phosphorus, and Potassium that is imported to livestock and poultry farms. This webcast highlights a decision aid tool that follows the nutrients in feed all the way to the land application of manure for crop production. This presentation was originally broadcast on October 17, 2014. More… Continue reading “Linking Feed Management to Whole Farm Nutrient Management”